You can manage all your subscription by going to the Apple App Store on your iPad. See This Aple Support page for details. http://support.apple.com/kb/ht4098
Navigate to your purchases and locate the SOS subscription then turn off auto-renewal (this cancels your subscription).
Note: the subscription will only cancel once the current billing period has ended. As the SOS app is billed on a 12-month subscription, turning off auto-renew will only cancel your sub from renewing once the first 12 months are up. In order to cancel a subscription immediately and obtain a refund, see our related FAQ: